In known thermal printers where the shades or tones of printed dots are controlled, the voltage or current applied to heat-producing resistors or the time it is applied is varied. Of these devices, those printers where the voltage or current is controlled result in an increase in the power supply capacity and need a complicated circuitry to adjust the voltage or current. Also, known printers which control the time using analog devices, such as monostable multivibrators, tend to produce less accurate shades than printers generating dots which can have 8 to 16 discrete shade variations.